The present invention relates to a baby trailer to be pulled by a baby tricycle or the like and, more particularly, to a folding collapsible frame structure for baby trailer, which has the transverse front bar of the bottom frame unit thereof curved downwards so that a young child can conveniently move in and out of the baby trailer without kicking the transverse front bar and, which enables the trailer bar to be automatically locked when moved to the extended or received position.
Various folding collapsible baby trailers have been disclosed, and have appeared on the market. However, conventional folding collapsible baby trailers are still not satisfactory in function. When getting on/off the baby trailer, the young child's lets tend to kick or hook on the front transverse rod member. Further, because a lock pin is used to selectively lock the handle between the extended position and the received position, the user may forget to install the lock pin after the handle has been moved to the extended position. If the handle is not locked when pulling the baby trailer, an accident may occur
The present invention has been accomplished to provide a folding collapsible baby trailer, which eliminates the aforesaid problem. According to one aspect of the present invention, the folding collapsible baby trailer is  comprised of a bottom frame unit carrying a pair of wheels, two upright side frames respectively pivoted to the bottom frame unit at two sides, a transverse top bar detachably connected between the upright side frames at the top, and a trailer bar pivoted to a bottom U-lug at one side of the bottom frame unit and selectively locked between the extended position and the received position. The bottom frame unit has a transverse front bar, which has a middle section curved downwards without hindering movement of a young child in and out of the baby trailer. According to another aspect of the present invention, the trailer bar is pivoted to the bottom U-lug at the bottom frame unit, and selectively locked between the extended position and the received position by a spring-supported locking member, which is coupled to the trailer bar by a slip joint.